I have some static content on my web site that I have set up caching for (using Asp.NET MVC). According to Firebug, the first time I open the page, Firefox sends this request:

GET /CoreContent/Core.css?asm=0.7.3614.34951
Host: 127.0.0.1:3916
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Accept: text/css,*/*;q=0.1
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://127.0.0.1:3916/Edit/1/101
Cookie: .ASPXAUTH=52312E5A802C1A079E2BA29AA2BFBC5A38058977B84452D62ED52855D4164659B4307661EC73A307BFFB2ED3871C67CB3A9AAFDB3A75A99AC0A21C63A6AADE9A11A7138C672E75125D9FF3EFFBD9BF62
Pragma: no-cache
Cache-Control: no-cache

Which my server replies to with this:

Server: ASP.NET Development Server/9.0.0.0
Date: Mon, 23 Nov 2009 18:44:41 GMT
X-AspNet-Version: 2.0.50727
X-AspNetMvc-Version: 1.0
Cache-Control: public, max-age=31535671
Expires: Tue, 23 Nov 2010 18:39:12 GMT
Last-Modified: Mon, 23 Nov 2009 18:39:12 GMT
Vary: *
Content-Type: text/css
Content-Length: 15006
Connection: Close

So far, so good. However, if I refresh Firefox (not a cache-clearing refresh, just a normal one), during that refresh cycle Firefox will once again go to the server with this request:

GET /CoreContent/Core.css?asm=0.7.3614.34951
Host: 127.0.0.1:3916
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Accept: text/css,*/*;q=0.1
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://127.0.0.1:3916/Edit/1/101
Cookie: .ASPXAUTH=52312E5A802C1A079E2BA29AA2BFBC5A38058977B84452D62ED52855D4164659B4307661EC73A307BFFB2ED3871C67CB3A9AAFDB3A75A99AC0A21C63A6AADE9A11A7138C672E75125D9FF3EFFBD9BF62
If-Modified-Since: Mon, 23 Nov 2009 18:39:20 GMT
Cache-Control: max-age=0

to which my server responds 304 Not Modified.

Why does Firefox issue this second request? In the first response, I said that the cache does not expire for a year (I intend to use query parameters whenever things change). Do I have to add another response header to prevent this extra roundtrip?

Edit: It does not matter whether I press refresh, or whether I go to the page again (or a different URL, which references the same external files). Firefox does the same again. Also, I don't claim this to be a bug in FF, I just wonder if there is another header I can set which means "This document will never change, don't bother me again".